I think Watase Yuu (Absolute Boyfriend, Alice 19th, Ayashi no Ceres, Imadoki) and Fukushima Haruka (AAA, Cherry Juice, Orange Planet) are some of the best. But there are so many great mangakas out there!

Matsuri Hino b/c I like her works and her drawing is cute(Vampire Knight, Meru Puri)
Arina Tanemura b/c her drawing is nice and her mangas are interesting(Shinshi Doumei Cross, Full Moon, ION, KKJ)
Maki Minami b/c I also like her drawing and her works are fun to read(S.A, Kimi wa Girlfriend)
Bisco Hatori b/c her stories are funny and romantic(OHSHC, Millenium Snow)
Natsumi Ando b/c her mangas tells how to love people and a free recipe(Kitchen Princess)
Hayakawa Tomoko b/c her work is so so funny(Wallflower)
Fukushima Haruka b/c dunno... just love her stories(AAA, Cherry Juice)